The prior art discloses an electronically controlled, variable-effort power steering system wherein the system varies the assist level by restricting the hydraulic fluid flow rate to a vehicle steering rack. Flow rate control is achieved by controlling the current through a solenoid valve which, in turn, operates a flow control valve through which the hydraulic fluid flows. Hydraulic fluid flow rate curves (such as flow rate vs. vehicle speed and steering wheel rotation rate) are typically defined during vehicle development and are incorporated in the software design of the vehicle's electronic control module or unit to generate corresponding current. U.S. Pat. No. 4,629,025 entitled "Flow Regulated Power Assist Steering System With Evasive Maneuver Capability" and issued Dec. 16, 1986 is an example of such prior art.
One problem with such prior art is that such control systems provide open-loop control of the solenoid valve. Consequently, there is no capability to modify or adjust the current control signals to the solenoid valve as defined by the hydraulic fluid flow rate curves programmed within the electronic control unit. The ability to modify the current control signal is highly desirable to compensate for changes in temperature and for variations in the coils of one actuator relative to another (for example, changes in electrical resistance). Variations also occur due to mechanical differences and other differences within the various solenoid valves.
Consequently, there is a need to accurately regulate the solenoid valve current over operating conditions including current fluctuations in the solenoid valve due to aging, variable battery voltage and temperature.
Closed loop feedback control systems are well known to correct system errors between desired output signals and actual outputs. For example, U.S. Pat. Nos. 4,639,651 and 4,666,013 disclose power steering systems which include feedback circuits. In the '651 patent armature current is fed to an electric motor which is used to provide power assist and is detected and fed back through an amplifier and a filter. The detected current is utilized in controlling a clutch for transmitting torque from the motor as an auxiliary torque to a steering output shaft.
In the '013 patent, sensed yaw rate or lateral acceleration is utilized as a feedback signal to modify the power assist signal delivered to an hydraulic assist cylinder.
U.S. Pat. No. 4,498,036 discloses an adaptive feedback system which determines an error signal representing the difference between an actual and a desired control factor in order to reduce the difference between the actual and desired levels. However, the system is not disclosed in combination with a power steering device.
Other U.S. patents of a more general interest include U.S. Pat. Nos. 3,874,407, 4,354,224 and 4,439,716.